Client-Server architecture entails client computers such as personal computers, laptops, smart-phones and tablet devices connected to server computers over a network. The client requests information from the server over the network using an agreed upon protocol and the server transmits the requested information back to the client.
The World Wide Web (WWW) is an example of the client-server architecture that is in popular use today. It entails a web client such as a web browser and a web server that are connected over a global system of interconnected computer networks called the Internet. The Internet uses the Transmission Control Protocol (TCP) as it's standard guaranteed delivery Transport Layer protocol.
The Hypertext Transfer Protocol (HTTP) serves as a Request-Response protocol over the World Wide Web. The web browser (or client) often referred to as a user agent (UA) submits a HTTP Request message to the web server. The web server in turn responds with a HTTP Response message containing resources such as Hypertext Markup Language (HTML) files to the web browser. The HTTP is an Application Layer protocol for the World Wide Web and operates on top of TCP, which is a Transport Layer protocol. The HTTP messages also carry HTTP header fields that define the operating parameters of an HTTP transaction. Further, these HTTP headers fall into standard and non-standard (prefixed with X-) categories.
A TCP connection between the web client and the web server is a virtual circuit protocol that guarantees an ordered delivery of data between the two. This connection is established using a 3-way handshake between the web client and the web server. In practice there are data buffers on the web client, web server and intermediate routers and switches that can delay the transfer of data introducing latencies and impacting the performance of the TCP connection.